Yes, you have to know the Page/Post ID, but that is easy compared to trying to do this in WordPress. You would actually have to login to each user's ID yourself and re-create each comment, then change the time/date stamp, e-mail address, user name and possibly URL, if specified, because doing it under your own ID will use YOUR gravatar, which cannot be changed!!!
So, this plugin is, in short, a practical alternative to native WordPress (which is completely impractical in most circumstances).
